FIG. 3 shows a cross sectional view of an exemplary conventional fluorescent display device 10. The fluorescent display device 10 includes a glass back plate 11, an anode substrate 12 having an anode conductor 16 formed on the anode substrate 12 and a phosphor layer 17 deposited on the anode conductor 16, and filaments 131 and 132 installed under the phosphor layer 17.
In the conventional fluorescent display device 10, light is emitted from the phosphor layer 17 by impinging thereon a low speed electron beam. In this case, the low speed electron beam can reach only a few angstroms from a surface of the phosphor layer 17. Therefore, the surface condition of the phosphor layer 17 has a great impact on the luminescence property of the fluorescent display device 10.
A typical process of fabricating a fluorescent display device includes the steps of: calcining an anode substrate, on which an anode conductor having thereon a phosphor layer is arranged, at a temperature ranging from 350° C. to 550° C.; sealing up the fluorescent display device with an envelope at a temperature ranging from 450° C. to 550° C.; and evacuating the interior of the fluorescent display device to a high vacuum at a temperature ranging from 300° C. to 400° C.
In each of the steps preceding the step of evacuating the display device to a vacuum, since the surface of a phosphor layer is vulnerable to damage due to various environmental factors, the surface may be easily contaminated and deteriorated in its quality. Further, there is a problem that the luminescence property of the display device may become unstable when exposed to even a very small amount of moisture and/or residual gas remaining in the device.
For instance, the phosphor used in the fluorescent display device may be an oxide phosphor such as ZnO:Zn, sulfide or oxysulfide phosphor such as ZnS:Cu,Al. Such sulfide or oxysulfide phosphor can be affected by a very small amount of moisture or residual gas remaining in the fluorescent display device even at a high vacuum, such that the luminescence property of the display device may be deteriorated.
Among the above-described phosphors, with regard to a (Zn,Mg)O system phosphor of yellow luminous color, there has been disclosed a method for providing a fluorescent display device having a high luminance and a long life span by adding WO3 in an amount of 0.05 to 20.00 wt % to the phosphor to thereby remove the residual gas remaining on the surface of the phosphor.
Further, for a (Zn,Mg)O system phosphor mixed with ZnGa2O4 of yellow luminous color, there has been disclosed a method for providing a fluorescent display device which emits a white light and has a long life span without using a pollution material such as cadmium (Cd).
However, a fluorescent display device employing the (Zn,Mg)O system phosphor of yellow luminous color has a deficiency in that the luminance of the display device may be diminished if the device is left unused or unlighted for more than a month.
In order to solve such a problem, it has been proposed to add WO3 having a particle size of 0.2 to 0.34 μm in an amount of 0.01 to 10.00 wt % to the (Zn,Mg)O system phosphor of yellow luminous color, which has a particle size of 4 μm. The phosphor mixed with WO3 may be used as a phosphor paste in a fluorescent display device.
Table 1 shows a high temperature exposure characteristic and a luminance residual ratio of such a fluorescent display device after an operation time of 1,000 hours at room temperature.
Herein, the luminance residual ratio of a display device means the ratio of a residual luminance after an operation time of 1,000 hours at a constant operational condition over an initial luminance of the display device. The luminance residual ratio is required to be more than or equal to 70%.
In general, the luminance residual ratio of 70% is equivalent to the luminance of the display device after an operation time of 10,000 hours, which is about 50% of the initial luminance.
Further, the high temperature exposure characteristic means the ratio of a luminance of the display device to the initial luminance after an exposure of the display device in an atmosphere of 85° C. for 72 hours. It is preferable that the high temperature exposure characteristic be more than or equal to 80%.
The high temperature exposure characteristic of more than or equal to 80% means that, after the display device is left unlighted at room temperature for a month, the ratio of a luminance of the display device to the initial luminance is 80%.
TABLE 1WO3High temperatureLuminance residualexposureAmount added (wt %)ratiocharacteristic0.0182%63%0.0578%65%0.175%70%0.470%72%162%75%252%76%543%78%1020%79%
Although not shown in Table 1, when WO3 was added in the amount of 0.10 wt % to the phosphor, the luminance of the fluorescent display device was 200 cd/m2.
Referring to Table 1, the luminance residual ratio is in an inverse proportion to the high temperature exposure characteristic as the amount of WO3 added varies. That is, while the luminance residual ratio is improved as the amount of WO3 added decreases, the high temperature exposure characteristic is improved as the amount of WO3 added increases.
Therefore, there is a need to provide a fluorescent display device having both a sufficient level of luminance residual ratio and a desirable degree of high temperature exposure characteristic.